In a nuclear reactor, fissile substances such as uranium-235 are consumed by a fission reaction, new fissile substances such as plutonium-239 being yielded as uranium-238 undergoes neutron absorption. The conversion ratio of the reactor is the ratio, at the time of unloading a spent fuel assembly, of the amount of fissile substance yielded to the amount of fissile substance consumed. In a conventional light water cooled and moderated reactor the conversion ratio is about 0.5. With the general aim of conserving uranium resources, recently there has been interest in increasing the conversion ratio of reactors.
In particular, recently published JP-A-1/227993 discloses a fuel assembly construction designed to achieve a conversion ratio approaching unity. The fuel assembly comprises an array of fuel rods arranged in a particularly dense configuration so that the effective volume ratio of water to fuel, as an average over the assembly, is not more than 0.4. The reactor is a boiling water reactor. So, this densely-packed fuel assembly construction will provide for recovery of nearly as much plutonium-239 etc. as the amount of fissile substance (uranium-235, plutonium-239) consumed. The recovered plutonium can be used to enrich uranium from any suitable source, and this can be burned in a nuclear reactor.
In a reactor, however, there are many factors other than conversion ratio which are important. In particular, reactors must be safe not only during normal operation but also should some abnormal transient condition arise. In a conventional boiling water reactor, the effective volume ratio of water to fuel is usually about 2.0; much higher than in JP-A-1/227993. With a soft neutron spectrum, the void coefficient of uranium-plutonium mixed fuel (void coefficient=change of reactivity with change of void fraction of coolant) is much less (more negative) than the corresponding void coefficient for an enriched uranium fuel. When the fuel rod configuration is made more dense to raise the conversion ratio, as disclosed in JP-A-1/227993, we observe that the void coefficient of uranium-plutonium mixed fuel tends to increase and approach positive values. Indeed, the prior art core having effective water to fuel volume ratio of 0.4 has a positive void coefficient.
This has important implications as regards safety. The safety of a reactor in the event of some abnormal transient or accident can be assessed with reference to a power coefficient. The power coefficient is the rate of reactivity change with unit power change, and is expressed as a sum of the void coefficient and a Doppler coefficient which is a component indicating reactivity change with temperature.
In fact, the particular construction described in JP-A-1/227993 does have a negative power coefficient and hence is safe in principle, because the Doppler coefficient is sufficiently negative to compensate for the positive value of the void coefficient. For increased control of safety, however, it would be desirable not to have to rely on the Doppler coefficient, but to be able to reduce (i.e. make less positive or more negative) the void coefficient contribution to the power coefficient.
It is known that void coefficient of a reactor core can be kept down by constructing the core so that electrons can leak easily, since void coefficient in a core depends on a sum of the changes of neutron infinite multiplication factor and neutron leakage value. Leakage of neutrons can suppress neutron infinite multiplication factor since although increased void fraction increases the number of neutrons in the core, it also increases the amount of leakage. However, a core which allows neutrons to leak easily has serious disadvantages, namely a lowering of reactivity with the leakage of neutrons at steady state.